The use of composite materials to replace conventional materials such as steel, aluminum, concrete and wood is becoming more commonplace and includes large structures such as bridges, body panels in automobiles and ships, and structural supports. Often, these components are thick section composites with varying core thickness. One of the challenges presented by the expanded use of composites, is the difficultly in inspection to assess structural integrity, manufacturing quality and to identify defects.
There are more numerous flaws that can occur in composites during manufacture that can have a significant impact on service life and performance than occur in metals or concrete. Today, the primary methods of nondestructive inspection are visual and ultrasonic. The techniques used in ultrasonic inspections typically require expensive specialized equipment, a highly trained operator and take a significant amount of time to perform the inspection and analyze the results. Additionally, many of the best ultrasonic inspection techniques are not suitable for in-service inspections. Systems such as the mobile automated ultrasonic scanning system do allow for field inspections, but are slow and often only provide an assessment at a few discrete locations on the structure.
Conventional nondestructive inspection techniques have made strides in locating flaws in some composites in the plane of the material and in the thickness direction. Defects such as delamination, fiber misalignment, cracking, matrix crazing and many other characteristics may be accurately determined. However, in thick-cored structures, composites with a thickness of an inch or more, the ultrasonic inspection is either rendered useless or at most can assess damage near the surface and visual inspections are inadequate.
What is needed is a non-destructive system and method that can easily and accurately detect structural anomalies in thick section composite materials in numerous settings and applied to varied shapes.
In accordance with the invention there is provided a new nondestructive inspection method for inspecting structures. In a preferred arrangement the method includes marking a mesh of test points on the structure to be tested. Optionally, the mesh or grid of points is rectangular. A plurality of sensors are attached at various locations on the structure, preferably not on the mesh and away from structural edges. The structure is excited by imparting a vibration force in the structure twice at each test point, and the subsequent response of the structure from each force input is recorded. The frequency response function (FRF) from the vibration force excitation at each of the test points is determined and the frequency dependent Operating Deflection Shapes (ODS) are determined from the FRF""s at each frequency and each of the test points. The ODS is differentiated to convert the information into an Operating Curvature Shape (OCS) by applying the finite difference approximation to both the real and imaginary parts of the ODS. A gapped cubic polynomial may be fitted to the OCS of the 5 nearest linear neighbor points of the test points of the mesh with separate functions being fitted to the real and imaginary parts of the complex function where the center value of the OCS has been removed. A structural irregularity index may be calculated by calculating the difference between the experimental curvature and the values of the cubic polynomials at each frequency and each test point. The results of the structural irregularity index values across all frequencies at each of the test points are summed and the results may be plotted on a contour map.
Optionally, the ODS may be normalized to an rms value of 1 prior to differentiating to curvature.
In a preferred arrangement, the vibration force may be imparted by an impulse hammer or a mechanical shaker. In a preferred arrangement the impact force imparts vibration energy in a frequency range of one to one hundred times the fundamental frequency of the structure. Optionally, the frequency corresponds to the resonant frequencies of the structure or may be in either a selected range of frequencies or a random range of frequencies.
Optionally, the gapped cubic polynomial may be fitted to the OCS in only one direction to allow for a different view of the results. In another preferred arrangement the gapped cubic polynomial may be fitted to the OCS in two intersecting directions and the results merged.
Numerous types of sensors may be used in alternative arrangements such as fiber optic Bragg gratings, mems sensors, strain gages, or other types of acceleration or displacement transducers.
In another preferred arrangement of the invention the ODS is not normalized to an rms value of one and the rms value of the damage indices at each line is normalized to an rms value of 1.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims.